The composite coupling devices described and claimed in my aforementioned applications and my copending British application No. 43684/74, the disclosure of which is also incorporated by reference, comprise a "driver", or heat-recoverable member, made from a memory metal and a second, sleeve member, usually an insert member, which is so constructed, and/or fabricated from such a material, that it enhances the coupling of the composite device to a substrate or substrates. Typically, the "driver" member and the "insert" member are both generally tubular and the insert member is provided with teeth and/or is made from a material with desirable properties, e.g. electrical properties, having regard to the particular application of the composite coupling device.
As is explained in the above applications, "memory metals" are alloys which exhibit changes in strength and configurational characteristics on passing through a transition temperature, in most cases the transition temperature between the martensitic and austenitic states, and can be used to make heat-recoverable articles by deforming an article made from them whilst the metal is in its martensitic, low temperature, state. The article will retain its deformed configuration until it is warmed above the transition temperature to the austenitic state when it will recover towards its original configuration. The deformation used to place the material in the heat-unstable configuration is commonly referred to as thermally recoverable plastic deformation and can also, in certain cases, be imparted by introducing strains into the article above the transition temperature, whereupon the article assumes the deformed configuration on cooling through the transition temperature. It should be understood that the transition temperature may be a temperature range and that, as hysteresis usually occurs, the precise temperature at which transition occurs may depend on whether the temperature is rising or falling. Furthermore, the transition temperature is a function of other parameters, including the stress applied to the material, the temperature rising with increasing stress.
Amongst such memory metals there may especially be mentioned various alloys of titanium and nickel which are described, for example, in U.S. Pat. Nos. 3,174,851, 3,351,463, 3,753,700, 3,759,552, British Pat. Nos. 1,327,441 and 1,327,442 and NASA Publication SP 5110, "55-Nitinol-The Alloy with a Memory, etc" (U.S. Government Printing Office, Washington, D.C. 1972), the disclosures of which are incorporated herein by reference. The property of heat recoverability has not, however, been solely confined to such titanium-nickel alloys. Thus, for example, various copper-based alloys have been demonstrated to exhibit this property in, e.g. N. Nakanishi et al, Scripta Metallurgica 5, 433-440 (Pergamon Press 1971) and such materials may be doped to lower their transition temperatures to cryogenic regimes by known techniques. Similarly, 304 stainless steels have been shown to enjoy such characteristics, E. Enami et al, id at pp. 663-68. These disclosures are similarly incorporated herein by reference.
In general, the alloys are chosen to have transition temperatures between the boiling point of liquid nitrogen, -196.degree. C., and room temperature as the lowest temperature likely to be encountered in operation, i.e. between -196.degree. C. and -75.degree. C. in many aerospace applications. This enables the articles made from the alloys to be deformed to the configuration from which recovery is desired, and stored, in liquid nitrogen and yet insures that after heat recovery there is no danger of loss of mechanical strength during use by reason of the article encountering a temperature at which it reverts to the martensitic state.
However, storage of the deformed article in liquid nitrogen is inconvenient. Recently processes have been developed by which metallic compositions, particularly certain copper-based alloys, can have the transition temperature at which they revert to the austenitic state transiently elevated from the normal temperature at which this occurs to a higher temperature, typically above room temperature. Subsequent recovery requires that the article be heated. Such alloys are referred to as being "preconditioned. " Procedures by which they are preconditioned are described in U.S. applications by G. B. Brook et al having the same assignee as the present application, filed Feb. 19th 1975 entitled "Heat Treating Method", Ser. No. 550,847, "Mechanical Preconditioning Method", Ser. No. 550,555 and "Austenitic Aging of Metallic Compositions," Ser. No. 550,556, the disclosures of which are incorporated by reference.
As indicated above, by application of a preconditioning process to an alloy its transition temperature can be elevated. However, once recovery has been brought about by heating the article through its new transition temperature, the alloy's response to temperature change reverts to that it possessed prior to preconditioning. Accordingly, it remains austenitic until cooled to the temperature at which transition to martensite normally occurs, typically chosen to be at 0.degree. C. or below depending upon the temperature environment likely to be encountered.
A typical application for the composite couplings described in the aforementioned Martin applications is to join tubular or cylindrical substrates. Properly dimensioned, these couplings can be employed to join substrate that vary greatly in size. For example, they might find application in joining tubing sections that could be used for hydraulic systems in aircraft. They can also be used to join sections of pipe of very large dimension.
As indicated above, the drivers associated with the composite couplings described in the aforementioned Martin applications are generally tubular members. Despite their many useful applications, composite couplings employing them are limited by the amount of recoverable dimensioned change that can be imparted to the drivers which in the case of tubular drivers is on the order of 6-8%. Another shortcoming to the prior art devices is the requirement that the heat recoverable driver be fabricated with a fixed diameter limiting the variations in insert and substrate size it can accommodate.
Accordingly, it is an object of the present invention to provide composite couplings in which the driver is capable of undergoing a high percentage of recovery when compared to composite couplings in which the driver-insert combination are simple tubular members.
Another object of the present invention is to provide a composite coupling in which the driver is capable of being employed with inserts and substrates that vary in dimension.